CN115386836A - Burnable poison coating coated on surface of nuclear fuel pellet and application - Google Patents

Abstract

The invention provides a burnable poison coating coated on the surface of a nuclear fuel pellet and application thereof, which are used for controlling reactivity and temperature coefficient of a moderator. The burnable poison coating of the invention consists of CeB ₆ 、SiB ₆ And YB ₆ At least one of them, the relative density is 70% -97%, and the thickness is 5-20 micrometers. The burnable poison coating has little loss of quality after passing a tape peel test after undergoing at least 3 thermal shock tests at 600 DEG CAnd (4) losing. On the one hand, the coating has a higher B content, and a lower B content can be used ¹⁰ Realization of B enrichment degree or natural abundance and commercial high ¹⁰ B enrichment of ZrB ₂ The same function, possess the cost advantage simultaneously. On the other hand, the bonding force of the coating and the matrix core block is also better than that of ZrB ₂ Therefore, the reactor core has higher reliability in the service process and is more beneficial to the safety of the reactor core. The coating may be integrated into a burnable poison fuel element for use.

Description

A burnable poison coating coated on the surface of nuclear fuel pellets and its application

technical field

The invention belongs to the field of nuclear fuel, in particular to a combustible poison coating coated on the surface of uranium-containing fuel pellets and its application. The coating has the function of flattening the neutron fluence distribution of the core, realizing long-period reactivity control and moderator The function of negative temperature coefficient control can be integrated in the burnable poison fuel element and applied to the reactor core.

Background technique

The information disclosed in this background section is only intended to increase the understanding of the general background of the present invention, and is not necessarily taken as an acknowledgment or any form of suggestion that the information constitutes the prior art already known to those skilled in the art.

During the initial refueling or initial refueling cycle of a PWR, the initial residual reactivity of the core is large, and it is necessary to use chemical compensation poisons, burnable poisons or control rods to control these residual reactivity.

For burnable poisons, the main functions are reflected in two aspects: one is to obtain the maximum fuel utilization rate and reduce the fuel cycle cost. The ability of the burnable poison to absorb excess neutrons decreases steadily with the operation, so as to gradually and finally fully release the reactivity bound by the burnable poison during the burn-up process; the second is to provide good power distribution control capabilities to realize the burnable poison The best match between consumption and fuel burnup in terms of rate and spatial relationship.

Literature (CESanders, JC Wagner, Study of the effect of integral burnable absorbers for PWR burnup credit, 2002; M.O'Leary, MLPitts, Effects of Burnable Absorbers on PWR Spent Nuclear Fuel, Office of Scientific & Technical Information Technical Reports, 2000; for Pressurized Water Reactors, 2000) studied and analyzed the combustible poison nuclides. The combustible poison nuclides commonly used now mainly include ¹⁰ B, ¹⁵⁷ Gd and ¹⁶⁷ Er. ¹⁵⁷ Gd and ¹⁶⁷ Er are mainly doped in the nuclear fuel in the form of oxides, and the total toxic residue at the end of life is stable above 4% of the initial toxicity. The consumption of element B decreases steadily and matches well with the fuel burnup. There is almost no residual penalty, and the consumption rate always decreases smoothly, so it is widely used. Therefore, the combustible poison materials used in pressurized water reactors at home and abroad mainly include boron stainless steel and boron carbide-alumina. , borosilicate glass and zirconium boride coating.

At present, there are two structural forms of commercial integral burnable poisons: (1) The burnable poison absorber and the nuclear fuel powder are mixed together and co-sintered to form a composite fuel pellet containing the burnable poison absorber, such as Gd ₂ O ₃ or Er ₂ O ₃ is dispersed in UO ₂ fuel to form a sintered body; (2) Coating a layer of burnable poison coating on the surface of nuclear fuel pellets, such as coating ZrB ₂ on the surface of fuel pellets to form an integral fuel burnable poison. The ¹⁰ B enrichment in the commercial ZrB ₂ coating of the AP1000 series nuclear power model exceeds 50wt%, and the enriched boric acid is used as the raw material for production. The manufacturing cost is high, and the bonding force between the coating and the substrate needs to be further improved. Patent CN11157368A discloses a neutron absorber material with higher neutron reactivity and high boron loading. The neutron absorber material is a multi-B compound, and the chemical formula of the multi-B compound is MB _x , where x Not less than 6, the mass fraction of B is not less than 75%, M is Al, Mg, Si, Y elements whose thermal neutron absorption cross section does not exceed 1.5 barnes and their mixtures. This patent does not protect the application of the above materials to the surface of nuclear fuel pellets, nor does it verify whether the coating process on the surface of nuclear fuel pellets can be realized.

Contents of the invention

In order to solve the above problems, the present invention provides a combustible poison coating coated on the surface of uranium-containing fuel pellets and its application. Under the reactivity adjustment function similar to that of the _ZrB coating, long-term reactivity control and Moderator negative temperature coefficient control. Aiming at the high cost of commercial high ¹⁰ B enriched ZrB ₂ , a burnable poison coating with higher boron content is provided, which can reduce the enrichment of ¹⁰ B and even use natural abundant boric acid for production, reducing material costs.

In order to achieve the above object, the present invention adopts the following technical solutions:

A first aspect of the present invention provides a burnable poison coating coated on the surface of nuclear fuel pellets, comprising:

The burnable poison coating is composed of at least one of CeB ₆ , SiB ₆ , and YB ₆ , and the relative density is between 70% and 97%.

The invention develops a new type of absorber material with higher boron content, and uses boric acid raw material with natural abundance or low ¹⁰ B enrichment degree, which is of great value for reducing material cost. At the same time, the stability of the combination of the coating and the substrate is not lower than that of the commercial _ZrB2 coating.

The second aspect of the present invention provides a nuclear fuel pellet, the surface of which is coated with the above-mentioned burnable poison coating.

Compared with the neutron absorber materials with high boron loading in the prior art, the present invention has optimized three kinds of materials including CeB ₆ , SiB ₆ and YB ₆ , especially through the magnetron sputtering technology under special control. The coating of CeB ₆ , SiB ₆ and YB ₆ on the surface of nuclear fuel pellets has been realized, and it has been technically verified that the three neutron absorber materials have specific application forms coated on the surface of nuclear fuel pellets.

The third aspect of the present invention provides a nuclear fuel element, including: the above-mentioned nuclear fuel pellets.

Beneficial effects of the present invention

(1) A burnable poison coating coated on the surface of nuclear fuel pellets provided by the present invention: the burnable poison coating is composed of at least one of CeB ₆ , SiB ₆ and YB ₆ . The B densities of CeB ₆ , SiB ₆ and YB ₆ are 1.52g/cm ³ , 1.7g/cm ³ and 1.56g/cm ³ , which are 30% higher than ZrB ₂ (1.17g/cm ³ ), 45 % and 33%. When the linear density, relative density and thickness of the coating ¹⁰ B are close to each other, the requirement for the enrichment degree of ¹⁰ B can be significantly reduced. The price of boric acid used to prepare B-containing materials increases exponentially with the ¹⁰ B enrichment degree, so the use of the burnable poison coating of the present invention can significantly reduce the cost of raw materials. At the same time, the binding force between the coating and the core block of the matrix is also better than that of ZrB ₂ , which has higher reliability during service and is more conducive to core safety.

(2) The coating of the invention can flatten the neutron fluence distribution of the reactor core, and realize the long-period reactivity control of the reactor and the negative temperature coefficient control of the moderator.

(3) The coating of the present invention has good compatibility with the nuclear fuel pellet matrix at room temperature to 800°C.

Description of drawings

The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention, and the schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations to the present invention.

Figure 1 is a fuel element comprising fuel pellets covered with a burnable poison coating according to the present invention. 1 is an upper end plug, 2 is a spring, 3 is a cladding, 4 is a coated core block, 5 is a support block, 6 is a support tube, and 7 is a lower end plug.

Fig. 2 is a fuel pellet covered with a burnable poison coating according to the present invention. 8 is the nuclear fuel pellet, 9 is the burnable poison coating, and 10 is the gap between the coated pellet and the cladding tube.

Fig. 3 is the nuclear characteristic curves of SiB ₆ , YB ₆ , CeB ₆ and ZrB ₂ four burnable poison coatings with the same ¹⁰ B linear density, ZrB ₂ is used as a comparison. The reactivity value curves of SiB ₆ , YB ₆ , and CeB ₆ have the same relationship with the burnup as the ZrB ₂ curve, that is, the difference between the initial reactivity value and the ZrB ₂ coating with the same ¹⁰ B linear density is no more than 200 pcm, and the lifetime The reactivity penalty at the end of the period differs by no more than 30 pcm from a ZrB ₂ coating of the same ¹⁰ B linear density.

Figure 4 is a photo of the cross-sectional microstructure of UO ₂ pellets coated with CeB ₆ coating. The average film thickness is about 10 μm.

Fig. 5 is the grazing incidence X-ray diffraction spectrum of the UO ₂ pellet surface coated with CeB ₆ coating. Except for the diffraction peaks of _UO2 matrix, the rest are diffraction peaks of _CeB6 .

Figure 6 shows the tape peeling test results after the UO ₂ pellets coated with CeB ₆ coating were subjected to five thermal shock tests at 600°C, and ZrB ₂ was used as a comparison. The adhesion between the CeB ₆ coating and the UO ₂ substrate was good, and the tape peeling mass was 0.0002 g after multiple thermal shocks, which was lower than the mass loss of the ZrB ₂ coating (0.0004 g).

Detailed ways

It should be noted that the following detailed description is exemplary and intended to provide further explanation of the present invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

A burnable poison coating coated on the surface of nuclear fuel pellets comprises: the main component of the coating contains at least one element among the three elements of B, Ce, Si and Y. The coating can flatten the neutron fluence distribution of the core, and realize long-period reactivity control and moderator negative temperature coefficient control. The coating has good compatibility with the substrate at room temperature to 800°C. The relative density of the coating is between 70% and 97%. The coating thickness is between 1.5 and 20 μm. The ¹⁰ B linear density in the coating is between 0.02 and 0.1 mg/mm. The ¹⁰ B enrichment in the coating ranges from natural abundance ( ¹⁰ B abundance is 18.4wt%) to 30wt%. The initial reactivity value of the coating differs by no more than 200 pcm from a _ZrB2 coating of the same ¹⁰ B linear density. The end-of-life reactivity penalty differs by no more than 30 pcm from a _ZrB2 coating of the same ¹⁰ B linear density. After the coating has undergone at least three thermal shock tests at 600°C, the mass loss after passing the tape peeling test is less than 0.0006g.

Compared with the patent CN11157368A, the main difference of the present invention is that three material schemes of SiB ₆ , YB ₆ and CeB ₆ are selected, and the uniform coating on the surface of nuclear fuel pellets is realized through the magnetron sputtering process, which provides coating in Specific application scenarios on fuel pellets and integrated into fuel elements. The burnable poison coating described in the patent of the present invention achieves the purpose of using lower ¹⁰ B abundance or even natural abundance materials to equivalently replace high-abundance ZrB ₂ coatings in terms of reactivity adjustment function, while costing raw materials, coatings and It has advantages in matrix binding force.

In some embodiments, the specific process of magnetron sputtering includes: adopting CeB ₆ target material, cylindrical UO ₂ fuel pellets are placed in the center of the bottom plate of the rotating sample holder, and the upper bottom surface of the sample is covered with metal flakes; the Ar gas flow rate is adjusted to 60-80sccm, vacuum degree 0.6-0.8Pa, temperature of the sample chamber kept at 200-300°C, current of the magnetron to the CeB ₆ target kept at 120-150mA after arcing to coat the sample, coating 60-70 Hours later, the temperature of the sample chamber rose to 400-420°C, and the temperature was kept for 1-1.2 hours.

A nuclear fuel pellet containing a surface coating layer is coated with the surface coating having the above characteristics. The nuclear fuel pellets are uranium dioxide, thorium dioxide, plutonium dioxide, uranium dioxide-gadolinium trioxide, thorium dioxide-gadolinium trioxide, plutonium dioxide-gadolinium trioxide and mixtures thereof.

A fuel element containing nuclear fuel pellets with a surface coating layer includes the above-mentioned nuclear fuel pellets with a surface coating layer. The fuel element at least includes nuclear fuel pellets, structural materials for isolating the nuclear fuel pellets and coolant, and sealing materials for blocking openings of the structural materials. The thermal neutron microscopic absorption cross section of structural materials does not exceed 1.5 barnes, and no obvious chemical reaction occurs with nuclear fuel pellets and coolants between room temperature and 800 °C.

The present invention will be described in further detail below in conjunction with specific examples. It should be pointed out that the specific examples are to explain rather than limit the present invention.

Example 1:

An integral burnable poison coating on the surface of a PWR fuel pellet, assuming that the coating ¹⁰ has a linear density of 0.02 mg/mm, and when it is composed of SiB ₆ , YB ₆ and CeB ₆ respectively, the relative density of the coating is 70% and 97%, the coating thickness is shown in Table 1. When the composition of the coating is composed of two or three combinations of SiB ₆ , YB ₆ and CeB ₆ , the relative density of the coating is between 70% and 97%, and the thickness of the coating is between 1.7 μm and 4.0 μm. between.

Table 1 Relationship between relative density of burnable poison coating, ¹⁰ B enrichment degree and coating thickness ( ¹⁰ B linear density is 0.02mg/mm)

material ingredient Coating Relative Density (%) ¹⁰B enrichment (wt%) Coating thickness (μm) SiB₆ 70 18.4 4.0 SiB₆ 70 30 2.5 YB₆ 70 18.4 3.9 YB₆ 70 30 2.4 CeB₆ 70 18.4 4.0 CeB₆ 70 30 2.5 SiB₆ 97 18.4 2.9 SiB₆ 97 30 1.8 YB₆ 97 18.4 2.8 YB₆ 97 30 1.7 CeB₆ 97 18.4 2.9 CeB₆ 97 30 1.8

Example 2:

An integral burnable poison coating on the surface of a PWR fuel pellet, assuming that the coating ¹⁰ B has a linear density of 0.10 mg/mm and is composed of SiB ₆ , YB ₆ and CeB ₆ respectively, the relative density of the coating is 70% and 97%, the coating thickness is shown in Table 2. When the composition of the coating is composed of two or three of _SiB6 , _YB6 and _CeB6 , the relative density of the coating is between 70% and 97%, and the coating thickness is between 8.6μm and 20μm .

Table 2 Relationship between relative density of burnable poison coating, ¹⁰ B enrichment degree and coating thickness ( ¹⁰ B linear density is 0.10mg/mm)

Example 3:

An integral burnable poison coating on the surface of a PWR fuel pellet, assuming that the coating ¹⁰ has a linear density of 0.077mg/mm, when it is composed of SiB ₆ , YB ₆ and CeB ₆ respectively, the relative density of the coating is 70% and 97%, the coating thickness is shown in Table 3. When the composition of the coating is composed of two or three combinations of SiB ₆ , YB ₆ and CeB ₆ , the relative density of the coating is between 70% and 97%, and the thickness of the coating is between 6.6 μm and 15.4 μm. between.

Table 3 Relationship between relative density of burnable poison coating, ¹⁰ B enrichment degree and coating thickness ( ¹⁰ B linear density is 0.077mg/mm)

Example 4:

An integral burnable poison coating on the surface of PWR fuel pellets, the coating ¹⁰ B has a linear density of 0.077mg/mm and a relative density of 74%, and when the coating is composed of SiB ₆ , YB ₆ and CeB ₆ respectively, according to Table 4 provides the relative density and thickness of the coating, and the natural abundance ( ¹⁰ B abundance is 18.4wt%). The core properties of the above materials are basically equivalent to those of commercial _ZrB coatings, and have the same reactivity adjustment function. The nuclear characteristic curves are shown in Fig. 3, and the initial reactivity values of YB ₆ and CeB ₆ are sequentially higher by about 166 pcm and 186 pcm than the ZrB ₂ coating scheme. In the range of 15000-60000MWd/tU burnup, the reactivity penalties of YB ₆ and CeB ₆ coating schemes are higher than ZrB ₂ , 5pcm and 8pcm higher than ZrB ₂ on average. In this case, ^{the 10} B enrichment degree of the raw material boric acid for production is reduced from 24.4wt% to the natural abundance, which effectively reduces the raw material cost.

Table 4 Coating parameters with nuclear properties equivalent to commercial _ZrB2 coatings

material ingredient Coating Relative Density (%) ¹⁰B enrichment (wt%) Coating thickness (μm) ZrB₂ 74 24.4 14.3 SiB₆ 74 18.4 14.6 YB₆ 74 18.4 14.1 CeB₆ 74 18.4 14.5

Example 5:

An integral burnable poison coating on the surface of PWR fuel pellets, the coating ¹⁰ B has a linear density of 0.077mg/mm and a relative density of 74%, and when the coating is composed of SiB ₆ , YB ₆ and CeB ₆ respectively, according to The relative density and thickness of the coating provided in Table 5, the core properties of the above material with ¹⁰ B enrichment of 30wt% can be equivalent to the commercial ZrB ₂ coating, and achieve the same reactivity adjustment function. The ¹⁰ B enrichment degree of the raw material boric acid is reduced from 41wt% to 30wt%, which effectively reduces the raw material cost.

Table 5 Coating parameters with nuclear properties equivalent to commercial _ZrB2 coatings

material ingredient Coating Relative Density (%) ¹⁰B enrichment (wt%) Coating thickness (μm) ZrB₂ 74 41 8.8 SiB₆ 74 30 9.0 YB₆ 74 30 8.7 CeB₆ 74 30 9.0

From Examples 1 to 5, it can be clearly seen that the integral burnable poison coating on the surface of a PWR fuel pellet according to the present invention has the same reactivity control and moderator negative temperature coefficient control functions as commercial ZrB ₂ , while reducing the demand for ¹⁰ B enrichment, thereby effectively reducing the cost of raw materials.

Embodiment 6:

Coating of CeB ₆ coating was achieved on UO ₂ pellets by magnetron sputtering. The specific process is as follows: the CeB ₆ coating on the outer surface of the fuel pellet is prepared by a magnetron sputtering apparatus equipped with a radio frequency power supply, and a CeB ₆ target with a diameter of 50 mm is used. The cylindrical UO ₂ fuel pellets are placed in the center of the bottom plate of the rotating sample holder, and the upper bottom of the sample is covered with a metal sheet to avoid contamination by the coating. Adjust the Ar gas flow to 60 sccm, the vacuum to 0.6 Pa, and keep the temperature of the sample chamber at 200°C. After the arc started, the current of the magnetron to the CeB ₆ target was kept at 120mA to coat the sample. After 60 hours of coating, the temperature of the sample chamber was raised to 400°C and kept for 1 hour. Observed by SEM, the microstructure of the cross-section of the coated pellets is shown in Figure 4, and the average film thickness is about 10 μm. The grazing incidence X-ray diffraction spectrum of the coated pellet surface is shown in Figure 5, except for the diffraction peaks of _UO2 matrix, the rest are diffraction peaks of _CeB6 . The above examples show that the burnable poison coating of the present invention has manufacturing feasibility, and the coating has a high degree of crystallization. It should be noted that the coating thickness can be regulated by adjusting the preparation process parameters and the test implementation time.

Embodiment 7:

Peeling tests were performed on the _CeB6- coated _UO2 pellets with calibrated adhesive tape, and the _ZrB2- coated _UO2 pellets were used as a comparison. The photo of the tape after peeling is shown in Figure 6. After the peeling test of the CeB ₆ coated UO ₂ pellets, no obvious peeling substances were found on the tape, and after the peeling test of the ZrB ₂ coated UO ₂ pellets, obvious peeling substances were seen on the tape (shown in the circle in Figure 6). The above examples show that the combustible poison coating of the present invention is closely combined with the fuel pellet substrate, which is better than that of ZrB ₂ coated UO ₂ pellets.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (10)

1. A burnable poison coating for application to a surface of a nuclear fuel pellet, comprising:

the burnable poison coating consists of CeB ₆ 、SiB ₆ 、YB ₆ At least one of the components, and the relative density is between 70 and 97 percent.

2. A burnable poison coating applied to the surface of a nuclear fuel pellet according to claim 1, wherein the burnable poison coating has a thickness of between 5 and 20 μm.

3. A burnable poison coating applied to the surface of a nuclear fuel pellet according to claim 1, wherein the burnable poison coating comprises ¹⁰ The linear density of B is between 0.02 and 0.1 mg/mm.

4. A burnable poison coating applied to the surface of a nuclear fuel pellet according to claim 1, wherein the burnable poison coating comprises ¹⁰ The enrichment degree of B is between natural abundance and 30 wt%.

5. A burnable poison coating for coating onto the surface of a nuclear fuel pellet according to claim 1 wherein the burnable poison coating is prepared by a magnetron sputtering process comprising the steps of: by using CeB ₆ Target material, cylindrical UO ₂ The fuel pellet is arranged in the center of the rotating sample support bottom plate, and the upper surface and the lower surface of the sample are covered with the metal sheets; adjusting Ar gas flow to 60-80 sccm, vacuum degree to 0.6-0.8 Pa, sample chamber temperature to 200-300 ℃, and magnetron pair CeB after arcing ₆ The current of the target material is kept between 120 and 150mA to coat the sample, the temperature of the sample chamber is raised to between 400 and 420 ℃ after the sample is coated for 60 to 70 hours, and the temperature is kept for 1 to 1.2 hours.

6. A burnable poison coating applied to the surface of a nuclear fuel pellet according to claim 1 having an initial reactivity value equal to the value of the burnable poison coating ¹⁰ ZrB of B linear density ₂ The coating difference is not more than 200pcm;

or, the reactive penalty at the end of life is the same ¹⁰ ZrB of B linear density ₂ The coating difference is not more than 30pcm;

alternatively, the burnable poison coating has a mass loss of less than 0.0006g after passing a tape peel test after undergoing at least 3 thermal shock tests at 600 ℃.

7. A nuclear fuel pellet having a surface coated with a burnable poison coating according to any of claims 1 to 6.

8. A nuclear fuel pellet containing a surface coating according to claim 7 wherein the material of the nuclear fuel pellet is at least one of uranium dioxide, thorium dioxide, plutonium dioxide, uranium dioxide-gadolinium trioxide, thorium dioxide-gadolinium trioxide, plutonium dioxide-gadolinium trioxide, and mixtures thereof.

9. A nuclear fuel element, comprising: nuclear fuel pellet according to claim 7 or 8.

10. The nuclear fuel element of claim 9, further comprising: a structural material separating the nuclear fuel pellets from the coolant and a sealing material for plugging the openings of the structural material; the microscopic absorption cross section of thermal neutrons does not exceed 1.5 target' n, and no obvious chemical reaction occurs between the thermal neutrons and the nuclear fuel pellet and the coolant at room temperature and 800 ℃.

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